Capturing a Network Trace on Azure Container Apps

If you're running an application on Azure Container App, there may come a time when you need to troubleshoot network issues. One of the best ways to do this is by collecting a network trace. In this blog post, we'll walk you through the process of collecting a network trace on Azure Container App using the `tcpdump` tool.

Step 1: Accessing the Azure Container App Console

To access the Azure Container App Console, navigate to your container app in the Azure Portal. Once you've selected your container app, click on the "Console" option under the "Monitoring" blade. This will open up a terminal window in your browser that you can use to connect to your container.

Step 2: Installing `tcpdump`

In case `tcpdump` is not installed on your container, you can easily install it using the package manager for your distribution. For example, on Ubuntu or Debian-based systems, you can use the following command:

apt-get update && apt-get install -y tcpdump

On Alpine-based systems, you can use the following command:

apk update && apk add tcpdump

Step 3: Capturing network packets with `tcpdump`

The syntax to start capturing packets would look like this:

tcpdump -i <interface> -s <snaplen> -w <file>

Make sure to replace the placeholders with the appropriate values:

• <interface> specifies the interface to capture packets from. If you want to capture traffic on all interfaces don’t use this switch.
• <snaplen> the maximum number of bytes to capture per packet. 
• <file> the name of the file to write the captured packets to.

For example, if you want to capture packets on the eth0 interface, you can use the following command:

tcpdump -i eth0 -s 0 -w capture.pcap

In the example above, the captured packets will be written to a file named capture.pcap.

The -s 0 flag sets the snaplen to 0, which means that `tcpdump` will capture the entire packet. This is useful when you want to capture all the details of a packet, including the payload.

Tcpdump will capture packets in real-time and write them to the specified file. To stop capturing packets, press Ctrl + C.

Analyzing network traces with Wireshark

Once you've captured network packets with `tcpdump`, you can first download it to your local machine by following this guide, and then analyze them using a packet analyzer such as Wireshark. Wireshark is a popular open-source packet analyzer that runs on Windows, macOS, and Linux.

To open the file containing the captured packets in Wireshark, open Wireshark and select File > Open. Navigate to the directory containing the capture file and select it. Wireshark will open the file and display the captured packets.

You can use Wireshark to filter, sort, and analyze the captured packets. For example, you can filter the packets to show only packets sent to or from a specific IP address or port, or you can sort the packets by timestamp to see the order in which they were captured.

Conclusion

Tcpdump is a powerful command-line tool that can help you capture network packets in real time. By installing tcpdump on a Container App and using it to capture network traces, you can gain valuable insight into network connectivity issues or suspicious network traffic. Analyzing the captured packets with a packet analyzer such as Wireshark can help you identify the root cause of the issue and take steps to address it.